New Radiometric Ages of Igneous and Mineralized Rocks, Southern Toquima Range, Nye County, Nevada D.R

Home , Mount Jefferson (Nevada)

New radiometric ages of igneous and mineralized rocks, southern Toquima range, Nye county, Nevada D.R. Shawe, C.W. Naeser, R.F. Marvin, and H.H. Mehnert Isochron/West, Bulletin of Isotopic Geochronology, v. 50, pp. 3-7 Downloaded from: https://geoinfo.nmt.edu/publications/periodicals/isochronwest/home.cfml?Issue=50

Isochron/West was published at irregular intervals from 1971 to 1996. The journal was patterned after the journal Radiocarbon and covered isotopic age-dating (except carbon-14) on rocks and minerals from the Western Hemisphere. Initially, the geographic scope of papers was restricted to the western half of the United States, but was later expanded. The journal was sponsored and staffed by the New Mexico Bureau of Mines (now Geology) & Mineral Resources and the Nevada Bureau of Mines & Geology.

All back-issue papers are available for free: https://geoinfo.nmt.edu/publications/periodicals/isochronwest This page is intentionally left blank to maintain order of facing pages. NEW RADIOMETRIC AGES OF IGNEOUS AND MINERALIZED ROCKS, SOUTHERN TOQUIMA RANGE, NYE COUNTY, NEVADA

DANIEL R. SHAWE CHARLES W. NAESER U.S. Geological Survey, P.O. Box 25046, Denver Federal Center, Denver, CO 80225 RICHARD F. MARVIN HARALD H. MEHNERT

INTRODUCTION Biotites from the Belmont pluton (samples DRS-81-898 and DRS-81-128)gave K-Ar ages of 81 -80 Ma, which are Thirty-five new radiometric ages are presented here for in accord with previously published ages (recalculated, on igneous rocks and mineralized material from the southern the basis of the new decay constants—Steiger and Jager, Toquima Range, Nye County, Nevada,to provide additional 1977)of biotites from the Belmont lobe. These ages are in information on the igneous and mineralization history of the ferred to be reduced ages and indicate that metamorphism area. Other radiometric ages for rocks of the area have occurred during the emplacement of aplite-pegmatite dikes been published by Krueger and Schilling (1971), Silberman and quartz veins in the pluton. and McKee (1971), Edwards and McLaughlin (1972), The 76-Ma biotite age of an aplite dike (sample Marvin and others (1973), Silberman and others (1975), DRS-81-54) in the Pipe Spring pluton and the 76-Ma Marvin and Dobson (1979), Boden (1986), and Shawe biotite and feldspar ages of a granodiorite dike (sample and others (1986). DRS-81 -58A)that is satellitic to the Pipe Spring pluton are These earlier studies have established the following age probably primary ages and are concordant with ages of framework for rocks of the area; the generalized geology is mineral deposits in association with the Pipe Spring pluton. shown on figure 1. Emplacement of the Round Mountain The pluton itself has an age of 80 Ma. lobe of the granite of Shoshone Mountain (Round Mountain The calc-silicate-mineralized limestone of the Lower pluton) into Paleozoic marine sedimentary rocks between Cambrian Gold Hill Formation in the Manhattan district Round Mountain and Belmont took place at about 95 Ma, appears to have been affected by at least three hydro- and emplacement of the granite of Pipe Spring (Pipe Spring thermal events. The first event was related to the minerali pluton) south of Manhattan occurred at about 80 Ma. Age zation that occurred about 76 Ma. A subsequent minerali of emplacement of the Belmont lobe of the granite zation event at about 40-35 Ma reset coarse-grained of Shoshone Mountain (Belmont pluton) has not been K-feldspar (sample DRS-80-58A) so that it now gives an established. Emplacement of aplite-pegmatite dikes and age of 45 Ma. A later Miocene event produced mineralizing quartz veins into, and metamorphism of, the plutons took solutions that deposited gold-bearing quartz-adularia place at about 80 Ma for the Round Mountain and Belmont veinlets in the calc-silicate-mineralized limestone about plutons and 75 Ma for the Pipe Spring pluton. 1 7 Ma (sample DRS-80-58B, age data from Shawe and A granodiorite stock and an associated rhyolite dike others, 1986). swarm were emplaced into the Round Mountain pluton and A number of ages, which range from 28 to 25 Ma, indi adjacent Paleozoic rocks near Round Mountain at about 36 cate that volcanic activity related to the Big Ten Ma. An episode of tourmalinization and metal mineraliza Peak caldera occurred during the late Oligocene. The tion was associated with emplacement of the stock. emplacement of ash tuff (sample DRS-81-166) and latite Geologic and geochronologic data suggest the possible flows(?) and plugs (samples DRS-81-167, DRS-81-177, presence of a buried stock and associated gold deposits o and DRS-81-182) was followed by intrusion of a rhyolite similar age in the east part of the Manhattan district plug (sample DRS-81-196) and late ash flows. Detrital (Shawe, 1986a). xx x d a zircon and apatite from a black-sand layer (sample The tuff of Mount Jefferson and related tuff df Round DRS-81-170) in conglomerate near the top of the early Mountain were erupted from the Mount Jefferson caldera ash-tuff unit gives ages of 23 and 25 Ma, respectively, at about 27-26 Ma. The Round Rock Fornnation (Shawe, which are seemingly too young relative to the other deter 1986b; Shawe and Snyder, in press), a silicic ash-flow tuff mined ages. sequence, was,emplaced in the Manhattan caldera at about Similar igneous activity occurred elsewhere in the region. 25 Ma. Gold mineralization, possibly related to buried The tuff of Round Mountain (sample DRS-79-85), which intrusives of the same igneous episode, also formed at was probably derived from the Mount Jefferson caldera, 25 Ma. ^ ^ .. was deposited about 27 Ma. A rhyolite plug (sample A young episode of gold mineralization occurred at Man DRS-81-1 55) intruded the ring-fracture zone of the Man hattan at about 16 Ma. No igneous rocks have been corre hattan caldera 25 Ma. lated with this event, but coeval intrusive bodies are Andesite that intruded the Ordovician marine sedi inferred to be present in the subsurface in or near the south mentary rocks near the southeast margin of the Manhattan margin of the Manhattan caldera. caldera gave an apatite fission-track age of 14 Ma (sample DRS-81-75A). The large age uncertainty (6 Ma) and the known age sensitivity of apatite to thermal resetting make an interpretation of this date uncertain. This age is a cool RESULTS ing age, but whether or not it is related to a late-phase The K-Ar and fission-track ages reported here (see thermal event is unknown. sample descriptions; sample localities shown on figure 1) Volcanic ash from a layer perhaps ten meters thick ex add to our understanding of the aforementioned age posed about 3 km east of the Manhattan caldera consists of fresh colorless glass shards. Zircon from the ash (sample framework.

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FIGURE 1. Simplified geoiogic map of part of the southern Toquima Range, Nevada, showing locations of the Round Mountain, Manhattan, Beimont, and Jefferson precious-metai districts, and sample localities. Pzs, Paleozoic sedimentary rocks; Kg, Cretaceous granite; Tg, Tertiary granodiorite stock; Tv, Tertiary volcanic rocks; Qa, Quaternary aliuvium. Numbered dot, sample locality showing radiometric age in Ma (AD, aduiaria; AP, apatite; B, biotite; HB, hornblende; KF, K-feldspar; S, sanldine; Z, zircon).

[ISOCHRONANEST, no. 50, December 1987] DRS-81 -1 26) gave a fisslon-track age of 1 2 Ma. The small silicate-mineralized limestone of the Lower Cambrian analytical uncertainty (2.5 Ma), together with the fresh Gold Hill Formation. glassy character of the ash, indicate that this igneous rock (K-feldspar)45.3 ± 1.0 Ma is from an unknown source that is considerably younger than any source previously dated in the southern Toquima 3. USGS(DhDRS-80-58B K-Ar Range. Veinlet (38°31 '55"N, 117O03'03"W; S20,T8N,R44E; mine dump of the Union Amalgamated Mine, east part ANALYTICAL INFORMATION of the Manhattan gold district; Manhattan 7%' quad., Nye Co., NV). Analytical data: K2O = 10.74%, Radiometric ages reported in this paper were determined 10.74%, 10.50%, 10.26%; ^Âr* = 2.578 x in the laboratories of the U.S. Geological Survey in Denver, 10"^° mol/g; *°Ar*/Z)*°Ar = 84%. Comments: Gold- Colorado. K-Ar ages were calculated with the following bearing quartz-adularia veinlet in calc-silicate-mineral- constants: = 0.581 x IQ-'o/yr, gp = 4.962 x ized limestone of the Lower Cambrian Gold Hill 1 Q-io/yr, and ♦"K/K = 1.167 x 10"* mol/mol. Potassium Formation. content was determined by flame photometry; argon con (adularia) 16.9 ± 0.6 Ma tent was determined by mass spectrometry. The decay constant for the spontaneous fission of that was used 4. USGS(DhDRS-81 -54 K-Ar to calculate fission-track ages is 7.03 x 10""/yr. The Aplite dike (38°30'30"N, 1 1 6 « 59'54" W; thermai neutron dose (n/cm®) was determined by counting S36,T8N,R44E; 2.5 km SW of mouth of East Man the induced fission tracks present in a piece of muscovite hattan Wash; Belmont West 714' quad., Nye Co. which covered a standard giass (SRM 962) during irradia NV). Analytical data: (biotite) K2O = 8.67%,8.51% tion. The constants and the neutron dose calibration used 40Ar» = 9.609 x 10"^° mol/g; ôAr^/E^Âr = 89% in this study have yieided F-T ages concordant with K-Ar (K-feldspar) K2O = 14.60%, 14.58%; ^Âr* = ages on unheated samples. The zircons were dated using 14.80 X 10-^0 mol/g; ôAr^/E^Âr = 82%. Com the external detector method, and the apatites were dated ments: Biotite-bearing aplite dike (30-m thick) em- using the population method (Naeser, 1976). The follow placed in Cretaceous granite of Pipe Spring. The ing abbreviations are used: Ps = fission-track density biotite age is probably the age of emplacement; the (tracks/cm®), number of tracks counted enclosed in paren K-feldspar age is too young, probably as a result of theses; and Pi = induced track density (tracks/cm®), diffusive loss of radiogenic argon from the K-feldspar number of tracks counted enclosed in parentheses. The crystals. quoted uncertainties represent the estimated analytical (biotite) 76.1 ± 2.7 Ma error at two standard deviations (2w) for K-Ar and fission- (K-feidspar) 69.2 ± 1.6 Ma track ages. 5. USGS(D)-DRS-81-58A K-Ar, Fission-track Granodiorite dike (38°30'11 "N,11 6®59'07"W; SAMPLE DESCRIPTIONS S36,T8N,R44E; 2 km SSW of mouth of East Man hattan Wash; Belmont West lY^' quad., Nye Co., 1 USGS(D)-DRS-79-85 K-Ar, Fission-track NV). Analytical data: K-Ar—(biotite) K2O = 8.19%, So ih flow tuff (38"4r26-N, 117»05'25-W. 8.17%; ♦oAr* = 9.160 x 10"^® mol/g; ôAr^/E^Âr S30,T10N,R43E; 1.5 km SW of Round Mountain = 89%; (K-feldspar) K2O = 9.21%, 9.13%; ^Âr* gold mine open pit; Round Mountain 772' quad., Nye = 10.32 X 10-^0 mol/g; ^Âr^/E^Âr = 92%. Co., NV). Analytical data: K-Ar—(biotite) K2O - Fission-track—(apatite —50 grains) Ps = 0.279 x 8.15%, 7.90%; *°Ar* = 3 ''04 x 10" 10® tracks/cm^ (582); Pi = 0.288 x 10® tracks/cm^ 40Ar*/E*oAr = 69%; (sanidine) K2O - (599); d = 0.696 x 10^® n/cm^; U = 1 3 ppm. Com 5.99%; "Ar* = 2.345 x 10"^® mol/g; "ArVE Ar ments: Biotite-rich granodiorite dike satellitic to the = 81%. Fission-track—(zircon—6 grains) Ps - o.n Cretaceous granite of Pipe Spring in an area of small X 10® tracks/cm® (749); Pi = 8-58 ^ mineral deposits. The biotite and K-feldspar ages tracks/cm® (1033); d = 1.20 x 10'® n/cm ; U - probably represent the age of the dike. The apatite 230 ppm; (apatite-50 grains) Ps = 0.105 >< 1" age may reflect a 40-35-Ma thermal event that ac tracks/cm® (219); Pi = 0.154 x 10® tracks/cm® companied a postulated early Tertiary mineralization (321); d = 0.696 X 10'® n/cm®; U = 7.0 ppm. in the Manhattan district. Comments: Light-grayish-brown porphyritic ""hyolite K-Ar (biotite) 76.1 ± 2.7 Ma welded ash-flow tuff from Oligocene tuff of Round (K-feidspar) 76.5 ± 2.8 Ma Mountain, now considered to be part of the Okgocene Fission-track (apatite) 40.4 ±6.8 Ma tuff of Mount Jefferson (Shawe and others, 1986; 6. USGS(D)-DRS-81-75A Fission-track Boden, 1986). (blotlte) 26.7 ± 1.7 Ma Andesite (38°31'54''N, 116°57'56"W; (sanidine) 27.0 ± 1.0 Ma S24,T8N,R44y2E; 1.7 km NNE of mouth of East Fission-track (zircon) 26.0 ± 2.6 Ma Manhattan Wash; Belmont West IV2' quad., Nye Co., (apatite) 28.4 ±7.8 Ma NV). Analytical data: (apatite—50 grains) Ps = 0.016 X 10® tracks/cm^ (34); Pi = 0.048 x 10® 2 USGS(D)-DRS-80'58A K-Ar tracks/cmM100);d =0.696 x 10^®n/cm2;U = 2.2 * Mineralized limestone (38°3V55"N, 117°03'03"W; ppm. Comments: Olive-brown fine-grained andesite S20,T8N,R44E; mine dump of the Union Amalga from an irregular body emplaced along bedding in the mated Mine, east part of the Manhattan gold district; Ordovician Toquima(?) Formation. Manhattan yVz' quad., Nye Co., NV). Analytical data: (apatite) 14.1 ± 6.3 Ma K2O = 13.64%, 13.44%; ôAr* = 8.949 x IQ-ô mol/g; ^°Ar*/E^°Ar = 86%. Comments: Coarsely 7. USGS(D)-DRS-81-89B K-Ar, Fission-track crystallized K-feldspar in sulfide-mineralized, calc- Granite (38°32'37"N, 116°58'10"W;

[iSOCHRON/WEST, no. 50, December 1987] S13,T8N,R44y2E; adit dump 1.5 km NW of mouth of Oligocene Manhattan caldera. The zircon fission-track Bald Mountain Wash; Belmont West l^h' quad., Nye age appears slightly young. Co., NV). Analytical data: K-Ar—(biotite) K2O = K-Ar (biotite) 24.8 ± 0.9 Ma 9.24%, 9.23%; = 10.97 x mol/g; FIsslon-track (zircon) 20.1 ± 2.4 Ma ♦oAr^/EôAr = 88%; (K-feldspar) K2O = 14.56%, 14.56%; ♦oAr* = 15.39 x 10-'° mol/g; 11. USGS(D)-DRS-81-166 K-Ar, Fission-track ♦®Ar*/E*°Ar = 94%. Fission-track—(apatite—1 GO Ash tuff (38®30'13"N, 1 1 6 ° 53'52''W; grains) Ps = 0.031 x 10® tracks/cm^ (130); Pi = S34,T8N,R45E; 1 km north of Hunts Canyon, 400 m 0.076 X 10® tracks/cmM317); d = 0.696 x 10^® south of hill 7428; Belmont West 7y2' quad., Nye n/cm^; U = 3.5 ppm. Comments: Light-gray, coarse Co., NV). Analytical data: K-Ar—(biotite) K2O = grained two-mica granite from an immense granite 8.39%, 8.36%; ^®Ar* = 3.279 x 10"^® mol/g; fragment of the Belmont lobe of the Cretaceous *0Ar*/E*0Ar = 72%. Fission-track—(zircon) —6 granite of Shoshone Mountain, embedded in ash tuff grains) Ps = 2.64 x 10® tracks/cm^ (732); Pi = of the megabreccia of Sloppy Gulch (Shawe and 7.61 X 10® tracks/cmM1,057); d = 1.20 x 10^® Snyder, in press). The 81 -Ma biotite age is a minimum n/cm^; U = 200 ppm. Comments: Light-gray to age for the Belmont lobe. The K-feldspar age is too whitish biotite-bearing ash tuff from the lowest ex young and is probably due to loss of radiogenic argon posed unit of the Oligocene Big Ten Peak caldera by diffusion. The apatite fission-track age is a cooling volcanic section. age, perhaps related to a Miocene thermal event. K-Ar (biotite) 27.0 ± 1.0 Ma K-Ar (biotite) 80.7 ± 2.9 Ma FIsslon-track (zircon) 24.8 ± 2.6 Ma (K-feldspar) 71.9 ± 1.7 Ma FIsslon-track (apatite) 17.1 ± 3.9 Ma 1 2. USGS(D)-DRS-81-167 K-Ar, Fission-track Latite flow(?) (38® 30'1 3" N, 1 1 6° 53'52''W; 8. USGSID)-DRS'81'126 Fission-track S34,T8N,R45E; 1 km north of Hunts Canyon, 450 m Volcanic ash (38°34'26''N,1 IB^BB'BCW; south of hill 7428; Belmont West 7y2' quad., Nye S30,T9N,R45E; 500 m east of Silver Creek on the Co., NV). Analytical data: K-Ar —(biotite) K2O = north side of knob 7103; Belmont West IV2' quad., 8.57%, 8.56%; ^®Ar* = 3.325 x 10"^® mol/g; Nye Co., NV). Analytical data: (zircon—4 grains) Ps *oAr*/EôAr = 69%. Fission-track—(zircon —4 = 0.969 X 10®tracks/cmM112); Pi = 5.81 x 10® grains) Ps = 5.06 x 10® tracks/cm^ (749); Pi = tracks/cmM336); d = 1.20 x 10^®n/cmÛ = 150 12.84 X 10® tracks/cm^ (951); d = 1.20 x 10^® ppm. Comments: Very light gray volcanic ash con n/cm^; U = 340 ppm. Comments: Light-lavender- sisting of fresh colorless glass shards. gray porphyritic latite flow(?) that overlies the lowest (zircon) 12.0 ± 2.6 Ma exposed unit of the Oligocene Big Ten Peak caldera volcanic section. 9. USGS(DhDRS'81'128 K-Ar, Fission-track K-Ar (biotite) 26.8 ± 1.0 Ma Granite (38°34'34"N, 116057'52"W; S31,T9N,R45E; FIsslon-track (zircon) 28.2 ± 3.0 Ma 150 m south of hill 7167, 1.3 km west of Silver Creek; Belmont West 7y2' quad., Nye Co., NV). Ana 1 3. USGS(D)-DRS-81-170 Fission-track lytical data: K-Ar—(biotite) K2O = 8.02%, 7.99%; Black-sand layer (38°30'19"N, 116O54'00"W; ^®Ar' = 9.432 x 10"^® mol/g; ^°Ar*/EôAr = 89%. 534,T8N,R45E; 1 km north of Hunts Canyon, 300 m Fission-track—(zircon—5 grains) Ps = 6.05 x 10® SSW of hill 7428; Belmont West 7 y2' quad., Nye Co., tracks/cm^ (1,288); Pi = 6.56 x 10® tracks/cm^ NV). >4/7a//f/ca/c/afa; (zircon —6 grains) Ps = 2.95 x (698); d = 1.20 X 10^® n/cm^; U = 170 ppm; 10® tracks/cm^ (791); Pi = 9.29 x 10® tracks/cm^ (apatite—100 grains) Ps = 0.049 x 10® tracks/cm^ (1,247); d = 1.20 X 10^® n/cm^; U = 250 pm; (206); Pi = 0.114 x 10® tracks/cm^ (475); d = (apatite—50 grains) Ps = 0.060 x 10® tracks/cm^ 0.696 X 10^® n/cm^ U = 5.2 ppm. Comments: (126); Pi = 0.099 x 10® tracks/cm^ (206); d = Light-gray, coarse-grained, nonporphyritic two-mica 0.696 X 10^® n/cm^; U = 4.5 ppm. Comments: granite of the Belmont lobe of the Cretaceous granite Black-sand layer in fluvial conglomerate bed near top of Shoshone Mountain. The 80-Ma biotite age is a of ash-tuff unit that is the lowest exposed unit of the minimum age for the Belmont lobe. The zircon and Oligocene Big Ten Peak caldera volcanic section. The apatite ages are cooling ages, perhaps related to zircon age appears slightly young. thermal events younger than 80 Ma. (zircon) 22.7 ± 2.2 Ma K-Ar (biotite) 80.0 ± 2.9 Ma (apatite) 25.4 ± 6.9 Ma FIsslon-track (zircon) 65.9 ± 6.7 Ma (apatite) 18.0 ± 3.6 Ma 14. USGS(D)-DRS-81-1 77 K-Ar, Fission-track Latite plug(?) (38®27'58"N, 1 1 6 ° 53'1 2" W; 10. USGS(D)-DRS'81-155 K-Ar, Fission-track 535,T8N,R45E; 500 m north of Hunts Canyon, 400 Rhyolite plug (38®36'54"N, 116®59'03"W; land m SW of hill 6850; Belmont West IV2' quad., Nye net unsurveyed; on ridge 1.5 km north of the south Co., NV). Analytical data: K-Ar —(biotite) K2O = fork of Silver Creek, 300 m east of pack trail in can 8.15%, 8.12%; ^®Ar* = 3.143 x 10"^® mol/g; yon; Belmont West 7y2'quad., Nye Co., NV). Analyti ^®Ar*/EôAr = 81%. Fission-track —(zircon —6 cal data: K-Ar—(biotite) K2O = 7.57%, 7.49%; grains) Ps = 2.86 x 10® tracks/cm^ (741); Pi = ^®Ar* = 2.704 x 10"^® mol/g; ^0Ar*/E^®Ar = 71%. 8.04 X 10® tracks/cmM 1,042); d = 1.20 x 10^® Fission-track—(zircon) —5 grains Ps = 2.11 x 10® n/cm^; U = 210 ppm; (apatite—50 grains) Ps = tracks/cm^ (488); Pi = 7.53 x 10® tracks/cm^ 0.082 X 10® tracks/cm^ (171); Pi = 0.090 x 10® (872); d = 1.20 X 10^® n/cm^; U = 200 ppm. Com tracks/cm^ (1 88); d = 0.696 x 10^®n/cm^;U =4.1 ments: Black glassy selvage of porphyritic rhyolite ppm. Comments: Light-gray porphyritic biotite-horn- plug emplaced in the ring-fracture zone of the blende latite plug(?) emplaced in the ring-fracture

[ISOCHRON/WEST, no. 50, December 1987] zone of the Oligocene Big Ten Peak caldera. The high REFERENCES crystal-defect density of the apatite grains severely limits the reliability of the apatite fission-track age. Boden, D. R.(1986) Eruptive history and structural development K-Ar (biotite) 26.6 ± 1.0 Ma of the Toquima caldera complex, central Nevada: Geological Society of America Bulletin, v. 97, p. 61 -74. Fission-track (zircon) 25.5 ± 2.6 Ma Edwards, G., and McLaughlin, W. A. (1972) Shell list no. 1 — (apatite) 37.9 ± 11.3 Ma K/Ar and Rb/Sr age determinations of California, Nevada, and Utah rocks and minerals: Isochron/West, no. 3, p. 1-7. 1 5. USGS(D)'DRS-81-182 K-Ar, Fission-track Krueger, H. W., and Schilling, J. H. (1971) Geochron/Nevada Latite plug(?) (38°30'24''N, 1 1 6° 52'49''W; Bureau of Mines K/Ar age determinations —list 1 : S35,T8N,R45E; 1.5 km north of Hunts Canyon, 500 Isochron/West, no. 1, p. 9-14. m NNE of hill 6850; Belmont West IV2' quad., Nye Marvin, R. F., and Dobson, S. W. (1979) Radiometric ages: Co., NV). Analytical data: K-Ar—(biotite) K2O = Compilation B, U.S. Geological Survey: Isochron/West, no. 26, p. 3-32. 8.43%, 8.42%; ^Âr* = 3.216 x 10"'° mol/g; Marvin, R. F., Mehnert, H. H., and McKee, E. H. (1973) A ôAr^/EôAr = 74%; (hornblende) K2O = 2.48%, summary of radiometric ages of Tertiary volcanic rocks in 2.40%, 2.41%; ôAr* = 0.9806 x x 10"^ mol/g; Nevada and eastern California, Part III—southeastern Nevada: *°Ar*/E*°Ar = 75%. Fission-track —(zircon —6 Isochron/West, no. 6, p. 1 -30. grains) Ps = 1.91 x 10® tracks/cm^ (513); Pi = Naeser, C. W.(1976) Fission track dating: U.S. Geological Survey 4.90 X 10® tracks/cm^ (658); d = 1.20 x 10^® Open-file Report 76-190, 58 p. n/cm^; U = 130 ppm; (apatite—50 grains) Ps = Shawe, D. R. (1986a) Complex history of precious metal 0.079 X 10® tracks/cm^ (164); Pi =0.117 x 10® deposits, southern Toquima Range, Nevada: U.S. Geological Survey Open-file Report 86-0459, 42 p. tracks/cmM243); d = 0.696 x 10^®n/cm2; U = 5.3 (1986b) Stratigraphic nomenclature of volcanic rocks ppm. Comments: Light-gray porphyritic biotite-horn- near Manhattan, southern Toquima Range, Nye County, blende latite plug(7) emplaced in the ring-fracture Nevada: U.S. Geological Survey Bulletin 1775-A, p. A1-A8 zone of the Oligocene Big Ten Peak caldera. The high [1987]. crystal-defect density of the apatite grains severely Shawe, D. R., Marvin, R. F., Andriessen, P. A. M., Mehnert, H. H., limits the reliability of the apatite fission-track age. and Merritt, V. M.(1986) Ages of igneous and hydrothermal K-Ar (biotite) 26.3 ± 0.9 Ma events in the Round Mountain and Manhattan gold districts, (hornblende) 27.8 ± 1.0 Ma Nye County, Nevada: Economic Geology, v. 81, p. 388-407. Shawe, D. R., and Snyder, D. B. (in press) Ash-flow eruptive Fission-track (zircon) 27.9 ± 3.4 Ma megabreccias of the Manhattan and Mount Jefferson (apatite) 28.1 ± 8.5 Ma calderas, Nye County, Nevada: U.S. Geological Survey Profes sional Paper 1471. 16. USGS(DhDRS-81-196 Fission-track Silberman, M. L., and McKee, E. H.(1971) K-Ar ages of granite Rhyolite plug(?) (38®31'33" N, 116°52'32"W; plutons in north-central Nevada: Isochron/West, no. 1, p. S23,T8N,R45E; 800 m east of hill 7034; Belmont 15-32. West IV2' quad., Nye Co., NV). Analytical data: Silberman, M. L., Shawe, D. R., Koski, R. A., and Goddard, B. B. (zircon —6 grains) Ps = 3.76 x 10® tracks/cm^ (1975) K-Ar ages of mineralization at Round Mountain and (1,010); Pi = 10.74 x 10® tracks/cm^ (1,442); d = Manhattan, Nye County, Nevada: Isochron/West, no. 13, p 1-2. 1.20 X 10" n/cm^; U = 280 ppm. Comments: Buff Steiger, R. H., and Jager, E. (1977) Subcommission on geo- porphyritic rhyolite plug(?) emplaced in ash-flow tuff chronology: Convention on the use of decay constants in geo- of the Oligocene Big Ten Peak caldera volcanic and cosmochronology: Earth and Planetary Science Letters, section. V. 36, p. 359-362. (zircon) 25.1 ± 2.3 Ma

[ISOCHRON/WEST, no. 50, December 1987]